1. Technical Field
The present disclosure generally relates to data center static synchronous compensators (DCSTATCOM) that are connected to a utility power grid at a point of common coupling (PCC) with data center load. More particularly, the present disclosure relates to compact multi-level medium voltage DCSTATCOMs that enable independent active (to provide uninterruptible power supply (UPS), grid energy storage, peak demand supply, Frequency support, power quality operations) and reactive power control (to provide PF corrector, grid voltage stiffness voltage support, grid voltage transient stabilizer operations) for data center loads that are connected to distributed energy sources (both regular and green). DCSTATCOM provides one innovative solution by integrating multiple functions as mentioned. It leverages same capital investment ($/kW cost of DCSTATCOM and MVUPS are in similar range) and generates better cost-benefit ratio incorporating multiple usages.
2. Background of Related Art
There is a large demand for storing digital data in data centers due to the emergence of Web-2.0-enabled businesses in the financial, e-commerce, pharmaceutical, and multi-media industries. The digital storage market doubles every 18 months, which translates to an annual growth rate of approximately 150% for the next 5 years.
Many data centers are equipped with on-site distributed power sources like fuel cells, solar, wind, geothermal, etc. for reliable power. These sources cause several specific problems including two-way power flow and two-way economic relationships. Balancing energy generation and consumption amidst a set of on-site distributed energy sources demands a significant balancing act. The availability and interconnection of multiple energy sources (grid and distributed) requires dynamic voltage regulation at the point of common coupling (PCC) to balance available supplies and load.
For reliable mission critical data centers, UPS is an integral part of data center design. The UPS and energy storage costs for such data centers are high and around $400/kW. Also, UPS is utilized less than 50% in Tier III and Tier IV data centers due to redundant design. To improve the overall Power Factor (PF) of the data center load at PCC to avoid the OPEX PF penalty charge and to reduce the CAPEX data center cost by eliminating UPS is achieved by connecting STATCOM at PCC. Also, by eliminating the UPS from the data center, data center design becomes very flexible because data center IT loads can be added or removed easily because they are not directly connected to the UPS. The active power of STATCOM acts as data center UPS at MV PCC. Also, this energy storage can act as grid energy storage when connected to distributed energy sources like Solar, Wind or FC.
FIG. 1 shows a system 100 with MV UPS and no STATCOM at PCC for supplying power to information technology (IT) and/or mechanical load 155 according to the prior art. The system 100 includes a utility/generator power supply system 195 and a UPS 115 that includes a step-up transformer 140. Under normal load conditions, power is supplied to the load 155 entirely by the utility supply 165. The utility supply 165 supplies an AC voltage ranging from about 3.3 kV to about 13.8 kV. The mechanical portion of the load 155 includes electrical power required to operate cooling equipment required to remove waste heat generated by the IT portion of the load 155.
A surge protector 180 is used to limit voltage spikes in the power supplied by the utility supply 165. A bypass line 162 allows maintenance tasks or other work to be performed on system 171-173 when ON/OFF switch of bypass line 162 (not shown) is closed and a static transfer switch (STS) 175 is opened. Line filters 170 are coupled to each AC line 171, 172, and 173 to reduce harmonics in the power supplied by the generator 160 or the utility supply 165. The STS 175 supplies power to a step-down transformer 150 when the STS 175 is closed. The step-down transformer 150 can convert the medium voltage supplied by the utility supply 165, e.g., 13.8 kV, to a low voltage, e.g., 400 V. The low voltage is then supplied to the load 155 having an appropriate current level.
When an interruption or disturbance in the power supplied by the utility supply 165 is detected, the STS 175 opens and the UPS system 115 starts supplying about 100% of the power to the load 155 via the UPS's step-up transformer 140. The UPS system 115 can supply power to the load 155 for a short period, e.g., approximately five minutes, but generally the generator 160 starts generating power if the interruption is more than a few seconds.
The UPS system 115 generates power from a low-voltage energy storage device 105, e.g., one or more low density lead-acid batteries B. The low voltage VB of the energy storage device 105 can range from about 300 V to about 600 V. The low voltage is then converted to a high voltage, e.g., approximately 700 V, by a bidirectional DC-DC converter 110. The bidirectional one-stage DC-DC converter 110 converts the low voltage DC to a high voltage DC. The high voltage DC is then converted to a low three-phase AC voltage, e.g., approximately 400 V, using a two-level inverter 120.
The AC voltage output from the two-level inverter 120 passes through filter 130, such as an inductor-capacitor (LC) filter, to a step-up transformer 140. The step-up transformer 140 converts the low AC voltage to a medium AC voltage, e.g., about 13.8 kV. The medium AC voltage output from the step-up transformer 140 is then provided to the step-down transformer 150, which converts the medium AC voltage to a low AC voltage, e.g., about 400 V, that is appropriate for the load 155.
Once the generator 160 has reached its reference speed and stabilized, transfer switch 190 shifts the primary power source from the utility supply 165 to the generator 160. During this shift, the output voltage of the UPS system 115 is synchronized to be in phase with the output voltage of the generator 160. Once the STS 175 is closed, a soft transfer from the UPS system 115 to the generator 160 is executed until the load 155 is entirely powered by the generator 160. The energy storage device 105 of the UPS system 115 is then recharged by the power generated by the generator 160.
After the power interruption or disturbance ends, the load 155 is shifted from the generator 160 to the UPS system 115 because the utility supply 165 may be out of phase with the generator 160 and the STS 175 shifts the primary power source to the utility supply 165. The output voltage of the UPS system 115 is then synchronized to be in phase with the output voltage of the utility supply 165. Once the output voltage of the UPS system 115 and utility supply 165 are synchronized, the load 155 is quickly transferred from the UPS system 115 to the utility supply 165. Then, the energy storage devices 105, e.g., batteries B, of the UPS system 115 are recharged from the utility supply 165 so that the UPS system 115 is ready for future interruptions or disturbances in the utility supply 165.
The step-up transformer 140 in the UPS system 115 meets the power requirements of the load 155; however, the step-up transformer 140 is a large and bulky component of the UPS system 115. As a result, the overall power density of the UPS system 115 is lower because the transformer 140 occupies a large amount of floor space, which, in some cities, can be quite expensive. The transformer 140 also introduces considerable losses (approximately 1 to 1.5% of the power) into the system thereby reducing the efficiency of the UPS system 115. Also, when the traditional sinusoidal pulse width modulation (PWM) technique is used to operate the inverters and an ON-OFF PWM technique for bi-directional single stage DC-DC converters 110 is used, current distortion increases. As a result, LC filters 130, which are expensive and bulky, are placed at the output of the two-level inverters 120 to reduce the current distortion or harmonics as demanded by the IT and/or mechanical load 155.
Alternately, a STATCOM (Static Synchronous Compensator) with step-up transformer (FIG. 2) is connected at PCC but it provides only reactive power compensation and therefore can provide only PF corrector operation at PCC and avoids PF penalty charge of the data center load. The data center still needs MV UPS to provide active power compensation in case of utility power disturbance to the IT load.
STATCOM is a member of the family of FACTS (Flexible AC Transmission System) controllers. FIG. 2 illustrates the application of an existing STATCOM at PCC along with MV UPS. Reactive STATCOM at PCC to an existing data center is used to compensate for reactive power. STATCOM is a shunt connected Voltage Source Inverter (VSI) and is connected to the grid through a smoothing reactor. It is to be noted that existing STATCOMs generate low voltage AC output through a two-level inverter. Therefore, it requires a step-up transformer at its output to match a utility voltage value (for example, 13.8 kV). However, the output step-up transformer is bulky, occupies extra space, and is inefficient. STATCOM, without an output transformer, has a small footprint as it replaces the transformer with a compact power electronic voltage converter. It significantly improves transient stability and regulates dynamic voltage at PCC (Point of common coupling). It also regulates both lag and lead reactive power. Therefore, STATCOM provides a stable voltage for a weak grid along with continuous reactive power regulation.
FIG. 2 illustrates a utility power feed 20′ supplied across a utility-load interface 5′ defining a utility side 51′ and a load side 52′ where load 55′ is a data center load as mentioned above. The utility power feed 20′ is electrically coupled to the data center load 55′. To compensate for reactive power losses caused by the reactive nature of the load 55′, a low voltage STATCOM 60 is coupled to the utility power feed 20′ at a point of common coupling 72 on the utility side 51′ via step-up transformer 68. STATCOM 60 includes a two-level DC-AC inverter 62. The two-level DC-AC inverter 62 is supplied power by a low voltage capacitor 64. The value of the capacitor 64 is small as it provides only reactive power compensation. The AC output of the two-level DC-AC inverter 62 is connected to a smoothing reactor 66 and is then supplied to a step-up transformer 68 whose AC output 70 is electrically coupled to the utility power feed 20′ at the point of common coupling 72 on the utility side 51′ of the utility-load interface boundary 5′.